The use of high-temperature superconductors (HTSC) in the electrical engineering encounters problems where the superconducting material must be present in the form of a wire or cable. The best HTSC's known to date are ceramic materials (a mixture of certain oxides) which are very brittle materials and do not lend themselves easily to a change of shape once formed. Therefore, the HTSC materials must be preformed into the desired shape and mechanically strengthened with a metallic part: a sheath, an envelope or capsule, a backing tape, a metallic core, etc. Furthermore, HTSC-metal composite wires or tapes are required for most superconducting applications. This composite design serves many important functions. Indeed, the metallic phase allows dissipation of local heat that could be induced by the motion of flux lines in alternative magnetic fields. The metallic phase gives the advantage of providing an alternative current route, although of a relatively high resistance, in case where the superconducting material exhibits a local loss of superconductivity (so-called normalization) which could lead to dramatic heating and a destruction of these composite conductors. Minimization of AC losses, reduction of mechanical stresses and protection against environment by the metal cladding are also important considerations.
These are basically two methods of forming superconducting materials: sintering and directional crystallization in the molten state. The first bulk HTSC's were synthesized by solid state reaction and sintering. Their superconducting properties and especially their critical current density (Jc) were quite low and very sensitive to small applied fields. This behaviour is attributed to the existence of weak links between superconducting grains. Moreover, these HTSC are anisotropic materials in which superconducting currents preferentially flow along the a-b planes. In sintered HTSC, low Jc are also attributed to misorientation of these a-b planes. Extensive efforts have been made to improve Jc and better results have been obtained using melting processes during which directional crystallization of the superconducting phase takes place; this is called texturing or melt-texturing. The improvement in Jc is believed to be due to grain alignment and better connectivity of the superconducting phase. Sintering is usually carried out at relatively low temperature and involves solid state diffusion while directional crystallization must be carried out at higher temperatures in the molten state or partially molten state.
Amongst the three families of HTSC known today, this invention relates particularly to the R-Ba-Cu-O family where R stands for Y or a rare earth element. These R-Ba-Cu-O superconductors are complex materials that undergo a peritectic reaction on heating, that is to say, instead of forming simply a liquid upon melting, they form liquid and a new solid. For instance, in the case of the YBa.sub.2 Cu.sub.3 O.sub.x (Y-123) superconductors, this peritectic decomposition may be expressed by the equation EQU YBa.sub.2 Cu.sub.3 O.sub.x .fwdarw.Y.sub.2 BaCuO.sub.5 +liquid(1)
This liquid is a Ba-Cu-rich solution and its exact composition depends on the temperature. In ambient air, this peritectic decomposition occurs at 1000.degree. C. The typical sintering temperatures are around 930.degree.-950.degree. C. while the temperature of directional crystallization must be higher than 1000.degree. C. However, this peritectic temperature also depends on the type of atmosphere in which the thermal treatment is carried out. For example, for the same YBa.sub.2 Cu.sub.3 O.sub.x superconductor, the temperature at which the peritectic decomposition occurs increases to 1010.degree.-1015.degree. C. in pure oxygen. Then, the exact composition of the liquid and its relative amount depend both on the temperature and the atmosphere.
This liquid phase formed during decomposition of the YBa.sub.2 Cu.sub.3 O.sub.x superconductor is extremely corrosive and reactive with a broad range of other materials, including metals (even noble metals), alloys and ceramics. Such reactivity does not come into play when sintering is used to form superconducting materials and combine them with a metallic envelope/backing tape since the temperatures used do not exceed the peritectic temperature of the superconducting material. An example of the prior art in this regard is European Patent Application No. 281 444 filed Feb. 05, 1988. A metal tube, the metal selected from a broad range of noble and other metals, is filled with ceramic powder selected to yield a superconducting ceramic, and then the tube is deformed so as to reduce its cross-section and subjected to a sintering heat treatment in the range from 700.degree. to 1000.degree. C.
Because of the high reactivity of the HTSC ceramics at or above their peritectic decomposition, many efforts have been directed at selecting a noble metal or an alloy of noble metals suitable for the processing of ceramic superconductors where the relatively high temperatures, above the peritectic temperature, are involved. M. Okada et al., "Texture Formation and Improvement of Grain Boundary Weak Links in Tape Shaped Wire Prepared by the Unidirectional Solidification Technique" (Mat. Res. Soc. Symp. Proc. Vol. 169, 1990, pp. 1283-1286) describe the performance of Au-sheathed Y-Ba-Cu-O superconducting tapes fabricated by drawing--rolling and subsequent unidirectional solidification. They did not mention any serious reaction problem between gold and the superconductor but this metal is very expensive and other authors showed that contamination or reaction problems occurred using gold as a metallic envelope (J. L. Porter et al., see below, and J. R. Verkouteren, SEM Analysis of Interactions between Pt, Au and AgPd Capsules and Barium-Yttrium-Copper Superconductors, Materials Letters, Vol. 8, No. 1.2, 1989, pp. 59-63).
J. L. Porter et al, "Reactivity of Ceramic Superconductors with Palladium Alloys" (J. Am. Ceram. Soc., 73 (6) 1990, pp. 1760-62) have tested palladium alloys for suitability as a non-reactive material for the processing of ceramic superconductors. The least reactive was found to be 70% Pd-30% Ag for YBa.sub.2 Cu.sub.3 O.sub.x HTSC's. However, a gap formed between the metallic foil and the superconductor after heat treatment at 1100.degree. C. Clearly, such a gap indicated that there was no good contact at the interface which is not acceptable from the viewpoint of electrical conductivity.
Silver alone cannot be used with R-Ba-Cu-O type HTSC (where R stands for Y or a rare earth element) for melt processing because its melting point, around 960.degree. C., is lower than the usual melt processing temperatures, above 1000.degree. C.
Binary silver alloys (Ag-Au and Ag-Pd) were found to react with the superconductor or the liquid formed during melt processing at high temperatures.